notebook static analysis refactoring

[edublancas]

We'll make a few changes to the way notebook static analysis behaves. It'll have three values now:

1. 'regular' - raise a warning at render time, raise an error at runtime (default)
2. 'strict' / True - raise an exception if there is an error at render time or runtime
3. 'disabled' / False - do not run static analysis

We make this change to allow users to run some of the commands that only require rendering because the current default value is too strict: if the pipeline has at least one notebook with a problem, users cannot even run ploomber status or ploomber plot

TO DOs:

• make the changes in the code
• update tests
• map boolean values to maintain backward compatibility
• improve error messages